There has recently been employed a papermaking process such as a so-called neutral or alkaline papermaking process which is carried out at pH 6 to 10, instead of an acidic papermaking process.
The neutral or alkaline papermaking process has advantages such that (1) a smaller energy for beating is used; (2) corrosion of machinery is reduced; (3) the white water system can be closed; (4) selection of fillers is not restricted and inexpensive calcium carbonate can be used; (5) deterioration of paper over time is reduce; (6) printability exhibited by the paper is good, and the like.
On the other hand, the neutral or alkaline papermaking process has drawbacks such that (1) expensive neutral sizing agents such as alkylketene dimers, alkenylsuccinic anhydrides, etc. must be used instead of inexpensive rosin sizing agents, and further, such agents are poor in anchoring onto the pulp; (2) fillers are poor in anchoring onto the pulp; (3) when the amount of calcium carbonate is increased, the strength of the paper is lowered; and the like.
It is known to use Mannich reaction products of an acrylamide polymer or acrylamide-acrylonitrile copolymer as an agent for reinforcing the paper strength in the papermaking under neutral conditions, but these agents are insufficiently anchored onto the pulp, and hence, satisfactory reinforcement of paper strength is not expected. Besides, there are also known polymers prepared by converting a part or whole of the tertiary amines in the above Mannich reaction products into quaternary groups. Although the products can increase the paper strength reinforcing effect in the papermaking step under neutral conditions in comparison with non-quaternary products, the improvement is still insufficient, and the anchoring effect of the neutral sizing agent is poor and hence the desired sizing effect is not achieved. Moreover, there is known a copolymer comprising predominantly (meth)acrylamide and a quaternary product of dimethylaminoethyl(meth)acrylamide, but it is still poor in the paper strength reinforcing effect, and the sizing agent is poor in anchoring effect. There is also known a copolymer comprising predominantly (meth)acrylamide and a quaternary product of dimethylaminopropyl(meth)acrylamide, but it is still poor in the paper strength reinforcing effect like the above polymer, and the sizing agent is poor in anchoring effect and hence a good sizing effect can not be achieved.
Besides, there is known a copolymer comprising predominantly (meth)acrylamide and diallyldimethylammonium chloride, but the sizing agent is poor in anchoring effect and hence the desired sizing effect can not be achieved.
Moreover, it is disclosed in Japanese Patent Second Publication (Kokoku) No. 12521/1972 to use a polymer having a partial quaternary group which is produced by copolymerizing diallylamine and a copolymerizable vinyl monomer e.g. (meth)acrylamide!, followed by modifying with an agent for converting into a quaternary group. However, such a polymer is particularly insufficient in the anchoring of the sizing agent and hence can not provide good sizing effect, while the paper strength reinforcing effect is improved.
Although there are also known one or more cationic polymers (e.g. polyamide/epichlorohydrin resin, etc.) and cationic starch, these are still not satisfactory in providing the desired effects. From these standpoints, it has been desired to develop an improved paper quality which is excellent in the yield of the product per sizing agent and filler and has excellent paper strength reinforcing effects.